解决Ubuntu 安装时Unable to fetch some archives connection failed [ip: 91.189.91.39 80]

在给ubuntu安装python opencv库时一直报错，说部分站点无法访问到。在网上查了很多方法都没有解决。最后修改sourses.list文件解决了问题

step1

找到sourses.list文件

在虚拟机下的etc/apt目录中

step2

找到国内的镜像资源
也可以在清华大学的镜像网站上找到
以下是18.04版本的

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-security main restricted universe multiverse

# 预发布软件源，不建议启用
# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ bionic-proposed main restricted universe multiverse
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step3

修改文件

$ sudo vim /etc/apt/sources.list
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将文件内容替换为镜像内容

step4

执行更新命令

$ sudo apt-get update
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更新完成后再次安装opencv，成功